Dell Dementions B110 The unit powers on all fans come on. Note: It was very dusty inside. On the screen the Dell boot up shows up for a second and goes off then comes back on then goes off again. It just keeps doing that. At the Power button there are two lights the one on the left starts orange then goes right to green and stays green. The light on the right will later come on green then go out and then come back on and then out etc.?

Could be shorting intermittently through dust on the front switch. Have you thoroughly cleaned the whole thing out properly and then tried?

Looks like a power problem or deribved from a power problem.

I think it may be the .PSU.. (overheating and the thermal cut-out its work).. Check the fan(s) in the PSU work OK - if not, clean them with paintbrush.
Best bet is to try another PSU (cheaper and a lot easier to replace than the mobo)

just suspishio suggested, check out your switch..if it's working fine.. make sure the contact of your switch is good.. and it doesn't toggle to on or off..